A system for long-reach lightwave data transmission through optical fibers has been described in U.S. patent application Ser. No. 10/289,944, filed Nov. 6, 2002 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM, which patent application is hereby incorporated herein by reference. This system uses an Optical Spectral Reshaper (OSR) to increase the extinction ratio at the output of a frequency modulated source, such as a directly modulated distributed feedback (DFB) diode laser, and reshapes the spectrum so as to increase the error-free transmission beyond the dispersion limit. An Optical Spectral Reshaper (OSR) is an optical element for which the transmission is a function of optical frequency. An OSR can alter the amplitude modulation of an input frequency modulated signal as well as alter the frequency profile of an input amplitude modulated signal. The transmitter described above is sometimes called a Chirp Managed Laser (CML™) by Azna LLC of Wilmington, Mass. The CML™ of Azna LLC has achieved error free transmission at 10 Gb/s through 200 km of single mode fiber having 3400 ps/nm dispersion.
The scheme for increasing the extinction ratio is shown in FIG. 1. Digital modulation of the laser changes its light output intensity, but also changes the optical frequency of its light output. This causes the 1 bit to be blue-shifted relative to the 0 bits. Since the transmission of the OSR is higher for higher frequencies near the edge of its transmission, the extinction ratio of the light passing through the OSR is increased when the spectrum of the laser output is aligned to be near the transmission edge of the OSR. This description assumes that the optical frequency shift is proportional to the light output of the laser, a frequency shift with this property is sometimes called “adiabatic chirp”. However, as shown below, a laser has other types of chirp which may cause distortions of the optical output from the desired waveform.
A directly current modulated laser diode, such as a DFB laser, exhibits three types of frequency modulation, or chirp, which accompany the intensity modulation: (1) adiabatic chirp; (2) transient chirp; and (3) thermal chirp. FIG. 2 shows these three types of chirp in relation to the modulated current applied to the laser and the output light intensity in FIG. 2.
The adiabatic chirp, which is proportional to the light intensity, is desirable and is central to the intensity shaping effect of the OSR.
The transient chirp, which has a short-term damped oscillatory behavior, and occurs at the 1-to-0 and 0-to-1 bit transitions, is usually undesirable, but can be controlled to manageable levels through proper biasing of the laser and proper selection of the filter bandwidth.
Thermal chirp is generally undesirable. It has the opposite sign to adiabatic chirp; i.e., an increase in current generates a blue-shifted adiabatic chirp, while it generates a red-shifted thermal chirp. In addition, while adiabatic chirp is nearly instantaneous and follows the output intensity, thermal chirp has a delayed response to the applied current, which increases exponentially in time. Thermal chirp is controlled by several time constants, which are relatively long in duration compared to the typical bit period of high speed digital signals, i.e., 100 ps for 10 Gb/s. The fastest time constant for thermal chirp is on the order of 25 ns for a typical DFB laser chip.
FIG. 3 shows an example of the output intensity and optical frequency of a directly modulated laser in response to modulation with a random digital bit sequence. Here only adiabatic and thermal chirp effects are included; transient chirp has been omitted. Thermal chirp is affected by the mark density of the bit sequence. For the purposes of this disclosure, mark density is the ratio of the number of 1s to the number of 0s that occur in a time period much longer than the bit period. For a truly random digital sequence, the mark density is ½ when averaged over a long period of time (e.g., seconds). However the sequence may have segments in time where the local mark density, measured over a shorter period (e.g., nanoseconds) is higher or lower than the average ½. When a DFB laser is modulated by a random sequence, a high density of 1's will tend to heat the laser since the average injection current is increased.
The temperature of the active region of the laser will decrease for a high density of 0s. The laser frequency changes with change in temperature because the refractive index of the semiconductor material is a function of temperature. Hence the temperature of the laser and its optical frequency tend to wander over time in response to short term changes in the mark density of the random sequence. The OSR converts this frequency wander to amplitude wander. Hence, thermal chirp causes the amplitude of the 1 and 0 bits to change slowly at the output of the CML™ depending on the mark density of the applied sequence. Hence, thermal chirp is generally undesirable in these systems.
This frequency wander can also cause another deleterious effect in data links with long lengths of optical fiber. Since the fiber is dispersive (i.e., since the velocity of light in the fiber varies with optical frequency), the frequency wander caused by thermal chirp can cause variations in the arrival time of the bits at the receiver. For normally dispersive fiber (i.e., positive dispersion), bits following a high density of is will arrive a little late because of the red shift induced by laser heating. On the other hand, bits following a high density of 0s will arrive a little early because of the blue shift induced by laser cooling. In other words, thermal chirp induces a pattern-dependent timing jitter in data links containing long lengths of dispersive fiber.